During metal processing, a metal oxide layer is often formed over all or part of the metal surface, impairing its appearance and/or suitability for further processing steps. One example is steel, including particularly stainless steels, where is often desired to produce a product having a bright, shiny finish, but the product can often be obtained that is covered wholly or partially with an oxide layer or flakes which impart a dull or blackened appearance. Other examples include nickel and chromium alloys, many of which contain a significant fraction of iron. Accordingly, it is desirable to remove or at least reduce the size and extent of the metal oxide layer. Processes to remove the oxide layer can be described as metal surface cleansing, or in some instances more particularly as metal pickling or polishing or desmutting depending on the specific treatment being carried out. Herein, the term pickling is employed to include all the above variations. In one class of processes, a solution of hydrogen peroxide is employed as oxidant, normally often an aqueous acidic solution to remove the oxide layer.
Steel pickling is normally carried out in an acidic medium. For many years, the preferred acidic medium comprised nitric acid, possibly containing hydrofluoric acid in addition, but during the last 20 years, alternative acid systems have been investigated in order to circumvent environmental problems caused by the emission of NOx from nitric acid-based baths. These alternative acid systems have included sulphuric acid, hydrofluoric acid, hydrochloric acid and mixtures of sulphuric acid with a haloacid such as hydrofluoric acid. In these alternative systems an oxidant is required for the pickling process, and in many instances, hydrogen peroxide has been proposed as that oxidant source. However, it will be understood that although hydrogen peroxide is the primary source of oxidation, the active species that carries out a significant fraction of the pickling treatment is believed to comprise ferric iron with the resultant formation in situ of ferrous iron so that the hydrogen peroxide often serves to oxidise the ferrous iron back to ferric iron species. Operation of the bath also results in further iron being dissolved, either from the oxide layer and/or from the steel surface itself, with consequential increase in iron concentration and eventually the need to replace or regenerate the pickling bath.
A number of publications have described the injection of a stream of air into strongly acid steel pickling baths, either with or without hydrogen peroxide. These publications include French Patent Specification 2 587 369, and European Patent Specifications 0505606 and 0582121. Although the injection of air not only serves to provide an alternative source of oxidant as well as agitating the bath liquor, such a process has a number of significant disadvantages. First, the passage of gas strips acidic and corrosive liquor from the bath, so that it suffers from environmental problems or requires subsequent scrubbing of the gaseous effluent, thereby incurring additional processing steps and cost. As a related consequence, there is a need also to replenish the bath more quickly than if the liquor were not being continually removed from the bath. Secondly, the evaporation of the liquor into the gas cools the bath, thereby increasing the heating costs. Thirdly, and perhaps more surprisingly, the use of an air stream tends to shorten the life of the bath before it needs to be replaced. That is because air bubbles in the stream have a tendency to lodge underneath the flakes of oxide, and as they expand they exert pressure on the flakes, separating them from the metal surface. Whilst it is an objective of a pickling process to remove the oxide layer, it is disadvantageous for too high a proportion to be removed within the pickling bath itself because the bath life is shortened. Instead, it would be advantageous for the flakes to remain adhering to the metal surface to a greater extent during the pickling process, but be dislodged during a subsequent high pressure washing process.
In Swedish Patent Application 8305648-1, there is disclosed an improvement compared with using an air stream, namely a process for reducing the extent of off-gas removal from an acidic pickling bath and particularly for pickling stainless steel, by recirculating the bath liquor through a recirculation line and injecting hydrogen peroxide into the line so that it is introduced into the bath through sprinklers, perforated pipes or nozzles placed at the bottom of the bath. The bath acids disclosed therein comprise sulphuric acid, or nitric acid or nitric/hydrofluoric acid. The same system is described in European Patent Application 776 993, through with the additional recognition that liquors used to pickle steel contain iron, which any expert recognises to be inevitable.
The aforementioned French Patent Application 2587369 European Patent 505606 and Application 582121 and British Patent Specification 2000196 all describe a process for steel pickling employing hydrogen peroxide as oxidant in which the introduction of hydrogen peroxide is controlled by the redox measurement of the bath to maintain a desired electropotential. This is an appropriate technique to adopt when the bath liquor contains and is intended to retain a substantial fraction of dissolved iron as ferrous species. However, since it is the ferric iron which the predominant active pickling species, such a process lessens the rate at which pickling can take place.